1. Field of the Invention
The present invention relates to a touch mode capacitive pressure sensor, and more particularly relates to the structure of a pressure sensor having high durability to pressure and a method for manufacturing the pressure sensor.
2. Description of the Related Art
A capacitive pressure sensor has a substrate on which is provided a diaphragm, which changes its shape in accordance with pressure, and a substrate which an electrode is provided on; these substrates are bonded together so as to face each other with a gap therebetween. Pressure is detected based on change in the capacitance between the diaphragm and the electrode. Wafers of silicon or glass can be used as the substrates where the diaphragm and the electrode are provided, enabling a large number of sensors to be formed on a wafer simultaneously, which is advantageous for inexpensive mass production.
FIG. 5A shows one example of such a capacitive pressure sensor, which is a touch mode capacitive pressure sensor disclosed in U.S. Pat. No. 5, 528,452. As shown in FIG. 5A, an electrode 1 comprises a metal thin-film and is provided on a glass substrate, and a dielectric film 2 is provided on the electrode 1. A diaphragm having at least a conductive surface is provided above and facing the dielectric film 2 with a slight gap 4 therebetween. As shown in FIG. 5B, when pressure is applied, the diaphragm 3 bends and touches the dielectric film 2 (this is termed “touch mode”).
The diaphragm 3 has a P+ layer which is made by high concentration doping of boron in n-type silicon, and, when the diaphragm 3 is regarded as one electrode, when detecting pressure, the electrode 1, the dielectric film 2, and the diaphragm 3 form a capacitor. Change in the area of the contact between the diaphragm 3 and the dielectric film 2 is detected as change in the capacitance between two electrodes (between the diaphragm 3 and the electrode 1), making it possible to measure the pressure on the diaphragm 3. The touch mode capacitive pressure sensor is more sensitive and has higher durability against overload pressure than other capacitive pressure sensors, and has many other superior characteristics, such as a linear output change of capacitance against applied pressure.
FIG. 6 shows the relationship between applied pressure and capacitance of the touch mode capacitive pressure sensor. Due to the characteristics of the touch mode capacitive pressure sensor, in the low pressure region (non-contact region) the output capacitance of the sensor does not change at all before the diaphragm touches the dielectric film. After the diaphragm touches the dielectric film, the capacitance of the sensor increases linearly (linear region) within a certain range with respect to pressure, and, when pressure increases more, sensitivity gradually decreases and change in the capacitance is saturated (saturation region).
A diaphragm using a single-crystal silicon is often formed by anisotropic etching, which utilizes differences in the etching rate due to the crystal orientation of the silicon, using an inorganic solution such as KOH, NaOH, or an organic solution such as ethlyene diamine pyrocatechol (EDP) and tetramethyl ammonium hydroxide (TMAH). Of the solutions for etching mentioned above, the KOH water solution is widely used in anisotropic etching of single-crystal silicon because it is inexpensive and has a higher etching rate than other solutions for etching.
In general, “etch stop technique” are used to form a thin diaphragm which has a few micron meters thickness only. One of the technique applies the effect that the etching rate in a highly doped P+ layer, for instance where boron concentration exceeds 1019 cm−3, is one tenth or one hundredth lower than that in a normal doped layer in silicon. By controlling the thickness of the diaphragm and the distance of the gap between the two electrodes, the linear region can be adjusted to match the required operating range of the sensor. For example, in the sensor for detecting tire pressure, the center of the operating range should be adjusted to approximately 10 kgf/cm2.
As described above, the conventional touch mode capacitive pressure sensor characteristically has high sensitivity and high durability against overload pressure, and can be made to operate stably within a required pressure range by changing the thickness of the diaphragm and the distance of the gap between the two electrodes.
However, some of applications require the sensor to withstand high pressure which greatly exceed the actual operating pressure range, e.g. four or five times higher than the upper limit of the measurable pressure range. Those diaphragm of the sensor may destroyed in such high pressure. The diaphragm breaks at the edge where most of the stress is produced when it bends and touches the dielectric film.